- Email: [email protected]
Laser doping: bipolar structures in silicon
Laser doping: bipolar structures in silicon I~G. IBBS, M.L. LLOYD Ultra-violet laser doping of silicon with boron and phosphorus, formed by the dissociation of trimethyl boron and phosphorus trichloride, has been achieved with sufficient control over concentration and depth to yield npn bipolar structures. KEYWORDS: lasers, doping, silicon, boron, phosphorus
Introduction Single step laser doping of silicon has been achieved by a number of groups, using the simultaneous laser generation of dopant species and rapid surface heating to promote diffusion into the substrate. The dopant is usually formed by pyrolysis~,2 or photolysis3,4 of some dopant precursor molecule that yields the dopant upon dissociation. A number of classes of molecule have been identified as suitable candidates for dissociation, the most common of which is probably the alkyl derivatives. Laser doping offers the possibilities of very rapid processing times -- of the order of microseconds for single implants; cost effectiveness -- when compared with the conventional techniques of ion implantation and oven annealing; and novel dopant depth distribution profiles, which are analogous to those found in pulsed laser annealings but without the compatibility problems allied to simple laser annealing of conventionally processed devices. In this group we have previously reported doping of silicon by boron formed from the dissociation of alkyl boron compounds such as triethyl boron (TEB) and trimethyl boron (TMB) using an ArF laser of wavelength 193 nm. The laser has a sufficient energy density at the substrate surface to cause a shallow (< 0.5/zm) melt phase in which the dopant diffuses rapidly. Subsequent cooling allows epitaxial regrowth to occur at the underlying single crystal phase, incorporating the dopant on electrically active sites. This process allows some degree of control of the final junction parameters. The substrate melt depth and therefore the deepest permissible junction depth, is sensitive to the irradiation energy density PE; varying from 0.05/xm to 0.5 ~m for 0.4 < PE < 2 J cm-2. Lowndes et aP have shown that the silicon optical damage threshold in the ultra-violet may be as high as 4 J cm -2 in a homogeneous beam, and calculations indicate that incident energies of 3-3.5 J cm-2 may produce melt depths of > 1.0/xm. Control of the dopant concentration, analogous to the 'dose' of ion implanted material, has been achieved using variable The authors are in the GEC Research Laboratories, Hirst Research Centre, Wembley, UK. Received 15 August 1983.
vapour pressures of dopant precursors, within the limits impose d by surface adsorption7. The dopant distribution profile may be tailored to some degree by using variations in the integrated melt duration with respect to the dopant diffusion rate caused by the number of laser pulses used to implant each site. In general, we have used up to 10 pulses for long integration times to produce profiles that are relatively insensitive to depth.
Experiment The details of the apparatus have been previously described4. A Lambda Physik EMGI01 rare gas halide (RGH) laser, normally operated with ArF, is focused into a stainless steel cell through a fused quartz window and on to the silicon substrate. The cell is connected to a diffusion pump with auxiliary vapour handling facilities so that the precursor vapours may be maintained at an arbitrary working pressure. Pressure monitoring is by an MKS Insts. baratron, and typical operating pressures are in the range 133 to 1.3 Pa. Laser pulses are individually monitored using an ultra-violet modified GEC model TF energy meter, have typical energies of 50 nO pulse-~ and are focused using a 20 cm lens to surface energy densities of --- 1 J cm-2. The laser pulse energy has a standard deviation of :i:10%.
Results Using variable irradiation energies and precursor molecules we have now demonstrated laser doping of both p and n species with sufficient control of depth and concentration to produce selective overdoping of npn bipolar structures. Figure I is a SIMS profile of such an npn structure. The substrate starting material is high resistivity (20--40 f~) <100> orientated n type (~10t4 phosphorus cm -~) silicon. Boron is formed by dissociation of a surface adsorbed layer of TMB, with the ambient vapour pressure in the cell < 0.01 Pa, and implanted to a depth ofN0.4 btm with a maximum concentration over the surface of"-4 × 10=' cm-3. The cell is subsequently pumped for several minutes to reduce the
0030-3992/84/010037-03/~03.00 © 1984 ButtenNorth Et Co (Publishers) Ltd OPTICS AND LASER TECHNOLOGY. FEBRUARY 1984
37
angle is tan -~ 0.00329, and the step interval of the probe is 2.5/~m, to give a depth resolution of 8 nm. Resistance data is then deconvolved to give carrier concentration.
20 -- f ' ~ , !
\
19 - -
"
et=
../'\
_./
16
i=
Fig, I
M It.0
]
0.2
I
04 Depth (l~m)
SIMSprofilesof boron (M 11.0) and phosphorus(M 31.0)
level of surface adsorbed TMB molecules, then filled to a pressure of 40 Pa phosphorus trichloride (PTC). The peak concentration of the phosphorus implant is ~,102° cm -3 -- significantly higher than the boron -but the implant depth is slightly shallower at only 0.3 pan due to partial absorption of the laser energy in the PTC vapour. This gives rise to two points of concentration overlap where the pn junctions are formed. It appears from Fig,. 1 that there is a shallow surface depletion of phosphorus. This arises as an artefact of the SIMS technique, due to a nonequilibrium ion yield caused by primary ion oxidation in the surface region'°. The effect of 'boil off of volatile components during pulsed laser annealing of, for example, GaAs has been reported to result in a shallow surface depletion region". It is not expected that this mechanism is responsible for the apparent loss of phosphorus in this case as there is no evidence of a near surface p-n junction in the spreading resistance profile shown in .Fig. 2.
At the surface in Fig,. 2 the n type material resistance is only N230 N, which in this case corresponds to a high carrier density of ~,,10~9 cm -~ indicating that significant overdoping has occurred whilst maintaining the material integrity. The resistance increases rapidly with depth until at 0.1/xm a shoulder is formed that represents compensation between n and p type carriers, shown by the discontinuity in carrier concentration. Between 0.1 and 0.225/~m the material is p type, with a peak carrier concentration of ,-,10~s cm-3. The deeper p-n junction at 0.225/xm shows up more markedly as a singularity in the resistance profile due to the low background dopant level in the starting material. The very clearly defined cusp in R at the junctions and the general absence of noise on the raw data, suggest that the lattice has suffered no major defects leading to junction leakage from the complementary dissociation products. This is not always the case, as we have demonstrated in the laser doping of silicon with boron formed from TEB4. There is evidence to suggest that the level of carbon implanted from TMB is very much lower than that for TEB, resulting in improved lattice qualityS; the effect of chlorine, from PTC or boron trichloride -- a further potential boron precursor, has not yet been fully measured. Attempts to detect C1 by SIMS have been unsuccessful but as the machine has only been used for detection of positive ions, this does not preclude the possibility that C1 is present. BCI3 has been successfully used as the dopant precursor in the formation of high efficiency (,-40% without AR coatings) solar cells9 indicating that the chlorine level in the silicon is very low -- or its presence does not adversely affect the material's parameters.
A 19 I I ,
/
Electrical characterization of the junctions has been performed using an ASR 100 C/2 two point spreading resistance probe on bevelled surfaces of samples nominally identical (within reproducibility) to those used in SIMS, as shown in Fig. 2. In this case the bevel
38
Resistance R t8
~
s j///
ii: o
The SIMS data, though indicative of overdoping~ are not sufficient to assign the junction positions unambiguously, as the technique provides no evidence of dopant activation, and therefore carder concentration, or lattice quality. Under normal pulsed ultra-violet laser annealing of ion implant damage, dopant activation yields have been shown to bell00% for device quality materiaP. However, single step laser doping introduces some new factors, such as complementary dissociation components from the precursor molecules, that may affect the activation level or introduce defects into the lattice.
k~.,..~ --
_J
Carrier concentration N
17 o 16
_J
/ J
15
2-
/
I-
I
0 o
o,I
I
14
I
02 0.5 Depfh (llm)
Fig. 2 -Spreading resistance (R) and carrier concentration (IV) profiles
OPTICS AND LASER TECHNOLOGY. FEBRUARY 1984
Conclusion W e have demonstrated the feasibility o f m a k i n g bipolar structures in silicon b y laser doping. T h e concentration, depth, a n d to some extent depth distribution, o f the d o p a n t m a y be controlled by variations in laser energy a n d d o p a n t precursor availability at the irradiated site. Current efforts are directed towards the study o f dissociation m e c h a n i s m s a n d products, a n d the possibility o f using m a s k e d projection techniques to f o r m complete devices or simple circuits where IV characteristics c a n be readily measured.
References Tamer, G.P., Tarruk D., Pollock, G., Preseby, P.., Press, P. 'Solar cells made by laser induced diffusion directly from PH 3 gas', Appi Phys Lett 39, (12), (1981), 967 2 Stuck, R, Fegarassy, E., Muller, J.C., Hodeau, M., Wa/tiaux, A., Siffert, P. 'Laser induced diffusion by irradiation of silicon dipped into an organic solution of the dopant', Appl Phys Leg 38, (9), (1981) 715
3 Detach, T.F., Ebrae~ D.J, aaem~, D.D., Severmi~, D.J., ~ P.M. 'Electrical properties of laser chemically doped silicon',Appl Phys ~ 39 (10), (1981), 825 4 Ibbs, ILG., Lloyd, M.L 'UV laser doping of silicon', Opt Laser TechnoL 15, "(1983) 35 5 For a general review see 'Laser solid interactions and laser processing', 1978 AIP Cont Proc No 50. Eds. S.D. Ferris, H3. Leamy and J.M. Poate. AIP, New York (1979)
6 Izvnui~ D.IL, Clehu~ J.W., Christie, W.IL, Eby, P.E., Jebn, G.E, N m y a , J., Wes~brook, P.D., Wood, P.F.,
7
8 9
I
OPTICS AND LASER TECHNOLOGY. FEBRUARY 1984
10 11
Nilson, J.A., Dass, S.C. 'Pulsed excimer laser annealing of ion implanted Si: characterisation and solar cell fabrication' Appl Phys Lett 41, (10), (1982) 938 lbbs, ILG., Lloyd, M.L 'Characteristics of laser implantation/doping'. Proc MRS 1982 - Syrup on Laser Diagnostics and Photochemical Processing of Semiconductor Devices. To be published by Elsevier (1983) Ibbs, K.G., Lloyd, M.L (To be published) DeuiscKT.F., Flat, J.C.C., Tinier, G.W, Chapman, P.L, Ehrlleh, D.J, Oqlood, P.M. 'Efficient solar cells by laser photochemical doping'ApplPhys Leg 38, (3) (1981). 144 Trigg, A.D., Ibbs, ILG., Sykes, D. 'Application of AES and SIMS to laser implanted boron in silicon'. SEM/83, 1, Ed O. Johari; SEM lnc Eisen, F.IL 'Laser and electron beam annealing of GaAs'. Laser and electron beam processing of materials, Ed C.W. White and P.S. Peercy, Academic, (1980)
39
Introduction Single step laser doping of silicon has been achieved by a number of groups, using the simultaneous laser generation of dopant species and rapid surface heating to promote diffusion into the substrate. The dopant is usually formed by pyrolysis~,2 or photolysis3,4 of some dopant precursor molecule that yields the dopant upon dissociation. A number of classes of molecule have been identified as suitable candidates for dissociation, the most common of which is probably the alkyl derivatives. Laser doping offers the possibilities of very rapid processing times -- of the order of microseconds for single implants; cost effectiveness -- when compared with the conventional techniques of ion implantation and oven annealing; and novel dopant depth distribution profiles, which are analogous to those found in pulsed laser annealings but without the compatibility problems allied to simple laser annealing of conventionally processed devices. In this group we have previously reported doping of silicon by boron formed from the dissociation of alkyl boron compounds such as triethyl boron (TEB) and trimethyl boron (TMB) using an ArF laser of wavelength 193 nm. The laser has a sufficient energy density at the substrate surface to cause a shallow (< 0.5/zm) melt phase in which the dopant diffuses rapidly. Subsequent cooling allows epitaxial regrowth to occur at the underlying single crystal phase, incorporating the dopant on electrically active sites. This process allows some degree of control of the final junction parameters. The substrate melt depth and therefore the deepest permissible junction depth, is sensitive to the irradiation energy density PE; varying from 0.05/xm to 0.5 ~m for 0.4 < PE < 2 J cm-2. Lowndes et aP have shown that the silicon optical damage threshold in the ultra-violet may be as high as 4 J cm -2 in a homogeneous beam, and calculations indicate that incident energies of 3-3.5 J cm-2 may produce melt depths of > 1.0/xm. Control of the dopant concentration, analogous to the 'dose' of ion implanted material, has been achieved using variable The authors are in the GEC Research Laboratories, Hirst Research Centre, Wembley, UK. Received 15 August 1983.
vapour pressures of dopant precursors, within the limits impose d by surface adsorption7. The dopant distribution profile may be tailored to some degree by using variations in the integrated melt duration with respect to the dopant diffusion rate caused by the number of laser pulses used to implant each site. In general, we have used up to 10 pulses for long integration times to produce profiles that are relatively insensitive to depth.
Experiment The details of the apparatus have been previously described4. A Lambda Physik EMGI01 rare gas halide (RGH) laser, normally operated with ArF, is focused into a stainless steel cell through a fused quartz window and on to the silicon substrate. The cell is connected to a diffusion pump with auxiliary vapour handling facilities so that the precursor vapours may be maintained at an arbitrary working pressure. Pressure monitoring is by an MKS Insts. baratron, and typical operating pressures are in the range 133 to 1.3 Pa. Laser pulses are individually monitored using an ultra-violet modified GEC model TF energy meter, have typical energies of 50 nO pulse-~ and are focused using a 20 cm lens to surface energy densities of --- 1 J cm-2. The laser pulse energy has a standard deviation of :i:10%.
Results Using variable irradiation energies and precursor molecules we have now demonstrated laser doping of both p and n species with sufficient control of depth and concentration to produce selective overdoping of npn bipolar structures. Figure I is a SIMS profile of such an npn structure. The substrate starting material is high resistivity (20--40 f~) <100> orientated n type (~10t4 phosphorus cm -~) silicon. Boron is formed by dissociation of a surface adsorbed layer of TMB, with the ambient vapour pressure in the cell < 0.01 Pa, and implanted to a depth ofN0.4 btm with a maximum concentration over the surface of"-4 × 10=' cm-3. The cell is subsequently pumped for several minutes to reduce the
0030-3992/84/010037-03/~03.00 © 1984 ButtenNorth Et Co (Publishers) Ltd OPTICS AND LASER TECHNOLOGY. FEBRUARY 1984
37
angle is tan -~ 0.00329, and the step interval of the probe is 2.5/~m, to give a depth resolution of 8 nm. Resistance data is then deconvolved to give carrier concentration.
20 -- f ' ~ , !
\
19 - -
"
et=
../'\
_./
16
i=
Fig, I
M It.0
]
0.2
I
04 Depth (l~m)
SIMSprofilesof boron (M 11.0) and phosphorus(M 31.0)
level of surface adsorbed TMB molecules, then filled to a pressure of 40 Pa phosphorus trichloride (PTC). The peak concentration of the phosphorus implant is ~,102° cm -3 -- significantly higher than the boron -but the implant depth is slightly shallower at only 0.3 pan due to partial absorption of the laser energy in the PTC vapour. This gives rise to two points of concentration overlap where the pn junctions are formed. It appears from Fig,. 1 that there is a shallow surface depletion of phosphorus. This arises as an artefact of the SIMS technique, due to a nonequilibrium ion yield caused by primary ion oxidation in the surface region'°. The effect of 'boil off of volatile components during pulsed laser annealing of, for example, GaAs has been reported to result in a shallow surface depletion region". It is not expected that this mechanism is responsible for the apparent loss of phosphorus in this case as there is no evidence of a near surface p-n junction in the spreading resistance profile shown in .Fig. 2.
At the surface in Fig,. 2 the n type material resistance is only N230 N, which in this case corresponds to a high carrier density of ~,,10~9 cm -~ indicating that significant overdoping has occurred whilst maintaining the material integrity. The resistance increases rapidly with depth until at 0.1/xm a shoulder is formed that represents compensation between n and p type carriers, shown by the discontinuity in carrier concentration. Between 0.1 and 0.225/~m the material is p type, with a peak carrier concentration of ,-,10~s cm-3. The deeper p-n junction at 0.225/xm shows up more markedly as a singularity in the resistance profile due to the low background dopant level in the starting material. The very clearly defined cusp in R at the junctions and the general absence of noise on the raw data, suggest that the lattice has suffered no major defects leading to junction leakage from the complementary dissociation products. This is not always the case, as we have demonstrated in the laser doping of silicon with boron formed from TEB4. There is evidence to suggest that the level of carbon implanted from TMB is very much lower than that for TEB, resulting in improved lattice qualityS; the effect of chlorine, from PTC or boron trichloride -- a further potential boron precursor, has not yet been fully measured. Attempts to detect C1 by SIMS have been unsuccessful but as the machine has only been used for detection of positive ions, this does not preclude the possibility that C1 is present. BCI3 has been successfully used as the dopant precursor in the formation of high efficiency (,-40% without AR coatings) solar cells9 indicating that the chlorine level in the silicon is very low -- or its presence does not adversely affect the material's parameters.
A 19 I I ,
/
Electrical characterization of the junctions has been performed using an ASR 100 C/2 two point spreading resistance probe on bevelled surfaces of samples nominally identical (within reproducibility) to those used in SIMS, as shown in Fig. 2. In this case the bevel
38
Resistance R t8
~
s j///
ii: o
The SIMS data, though indicative of overdoping~ are not sufficient to assign the junction positions unambiguously, as the technique provides no evidence of dopant activation, and therefore carder concentration, or lattice quality. Under normal pulsed ultra-violet laser annealing of ion implant damage, dopant activation yields have been shown to bell00% for device quality materiaP. However, single step laser doping introduces some new factors, such as complementary dissociation components from the precursor molecules, that may affect the activation level or introduce defects into the lattice.
k~.,..~ --
_J
Carrier concentration N
17 o 16
_J
/ J
15
2-
/
I-
I
0 o
o,I
I
14
I
02 0.5 Depfh (llm)
Fig. 2 -Spreading resistance (R) and carrier concentration (IV) profiles
OPTICS AND LASER TECHNOLOGY. FEBRUARY 1984
Conclusion W e have demonstrated the feasibility o f m a k i n g bipolar structures in silicon b y laser doping. T h e concentration, depth, a n d to some extent depth distribution, o f the d o p a n t m a y be controlled by variations in laser energy a n d d o p a n t precursor availability at the irradiated site. Current efforts are directed towards the study o f dissociation m e c h a n i s m s a n d products, a n d the possibility o f using m a s k e d projection techniques to f o r m complete devices or simple circuits where IV characteristics c a n be readily measured.
References Tamer, G.P., Tarruk D., Pollock, G., Preseby, P.., Press, P. 'Solar cells made by laser induced diffusion directly from PH 3 gas', Appi Phys Lett 39, (12), (1981), 967 2 Stuck, R, Fegarassy, E., Muller, J.C., Hodeau, M., Wa/tiaux, A., Siffert, P. 'Laser induced diffusion by irradiation of silicon dipped into an organic solution of the dopant', Appl Phys Leg 38, (9), (1981) 715
3 Detach, T.F., Ebrae~ D.J, aaem~, D.D., Severmi~, D.J., ~ P.M. 'Electrical properties of laser chemically doped silicon',Appl Phys ~ 39 (10), (1981), 825 4 Ibbs, ILG., Lloyd, M.L 'UV laser doping of silicon', Opt Laser TechnoL 15, "(1983) 35 5 For a general review see 'Laser solid interactions and laser processing', 1978 AIP Cont Proc No 50. Eds. S.D. Ferris, H3. Leamy and J.M. Poate. AIP, New York (1979)
6 Izvnui~ D.IL, Clehu~ J.W., Christie, W.IL, Eby, P.E., Jebn, G.E, N m y a , J., Wes~brook, P.D., Wood, P.F.,
7
8 9
I
OPTICS AND LASER TECHNOLOGY. FEBRUARY 1984
10 11
Nilson, J.A., Dass, S.C. 'Pulsed excimer laser annealing of ion implanted Si: characterisation and solar cell fabrication' Appl Phys Lett 41, (10), (1982) 938 lbbs, ILG., Lloyd, M.L 'Characteristics of laser implantation/doping'. Proc MRS 1982 - Syrup on Laser Diagnostics and Photochemical Processing of Semiconductor Devices. To be published by Elsevier (1983) Ibbs, K.G., Lloyd, M.L (To be published) DeuiscKT.F., Flat, J.C.C., Tinier, G.W, Chapman, P.L, Ehrlleh, D.J, Oqlood, P.M. 'Efficient solar cells by laser photochemical doping'ApplPhys Leg 38, (3) (1981). 144 Trigg, A.D., Ibbs, ILG., Sykes, D. 'Application of AES and SIMS to laser implanted boron in silicon'. SEM/83, 1, Ed O. Johari; SEM lnc Eisen, F.IL 'Laser and electron beam annealing of GaAs'. Laser and electron beam processing of materials, Ed C.W. White and P.S. Peercy, Academic, (1980)
39